automatic levels

the automatic level is easily recognized by its clean uncluttered appearance it does not have a tilting screw or a tubular bubular as the telescope is rigidly fixed to the tribrach and the line of sight is made horizontal by a compensator inside the telescope 

the basic concept of the automatic level can be liked to a telescope rigidly fixed at right angles to a pendulum . under the influence of gravity , the pendulum will swing into the vertical , as defined by a suspended plumb-bob and the telescope will move into a horizontal plane .

the collimation axis of the instrument will be inclined to the horizontal by a small angle to a pendulum . under the influence of gravity , the pendulum will swing into the vertical , as defined by a As the automatic level is only approximately levelled by means of its low-sensitivity circular bubble , the collimation axis of the instrument will be inclined to the horizontaly by a small angle a , so the entering ray would strike the erticule at a with a displacement of ab equal to f a . the compensator 

staff and cross-hairs

principle of compensator

situated at P would need to redirect the ray to pass through the cross-hair at b thus 

it can be seen from this that the positioning of the compensator is a significant feature of the compensation process for instance , if the compensator is fixed halfway along the telescope , then s = f/2 and n = 2 , giving  B = 2a .

there is a limit to the woring range of the compensate , about 20' hence the need of a circular bubble .

in order , therefore , to compensate for the telescope , the compensator requires a reflecting surface fixed to the telescope , movable surfaces influenced by the force of gravity and a dampening device (air or magnetic ) to swiftly bring the moving surfaces to rest and permit rapid viewing of the staff . such an arrangement is illustrated

suspended compensation

the advatages of the automatic level over the tilting level are :

(1) much easier to use , as it gives an erect image of the staff.

(2) rapid operation giving greater productivity ,

(3) no chance of reading the staff without setting the bubblecentral , as can occur with a tilting level .

(4) no bubble setting error .

A disadvantage is that it is difficult to use where there is difficult to use where there is vibration by lightly touching a tripod leg

(4) using an automatic level

the operation are identical to those for the titling level with the omission of operation (6) . some automatic levels have a button , which when pressed moves the compensator to prevent it stick this should be done just prior to reading the staff , when the cross - hair will be to move . another approach to ensure that the compensators is working is to move it slightly off level and if the reading on the staff is unaltered , thereby proving the compensator is working .

instrument adjustment

for equipment to give the possible results it should be frequently tested and , if necessary , adjusted surveying equipment receive continuous and often brutal use on construction sites . in all such case a calibration base should be established to permit weekly checks on the equipment.

two peg test

tilting level

the tilting level requires adjustment for collimation error only . collimation error occurs if the line of sight is not truly horizontal when the tubular bubble is centred , i.e . the line of sight is inclined up or down from the horizontal . A check known as the ' two - peg Test ' is used , the procesure being as follows

(a) set up the instrument midway between two pegs A and B set , say , 20 m apart and note the staff readings a1 and b1 , equal to , say , 1.500 m and 0.500 m respectively .

let us assume that the line of sight is inclined up by an angle of a ' as the lengths of the sights are equal (10m) , the error in each staff readding will be equal and so cancel resulting in a ' true ' difference in level between A and B .

H trus we know that A is truly lower than B by 1.000 m . we do not at this stage know that collimation error is present.

(b) move the instrument to C , wgich is 10 m from B and in the AB and observe the reading a2 and b2 equal to , say ,3.500 m and 2.000 m respectively . then

H = ( a2 - b2 ) = ( 3.500 - 2.000 ) = 1.500 m

now as 1.500 =/ the true , value 1.000 , it must be false .

H false = 1.500 m

and it is obvious that the intrument possesses a collimation error the amount and direction of wich is as yet still unknown , but which has revealed by the use of unequal sight lengths CB (10 m ) and CA (30 m ) , hade the two values for H been equal .then there would be no collimation error present in the instrument .

(c) imagine a horizontal line from reading b2 (2.000 m ) cutting the staff at reading a3 because A is truly 1.000 m , below B , the reading at a3 must be 2.000 + 1.000 = 3,000 m However , the acual reading was 3.500 m . and therefore the line of sight of the instrument was too high by 0.500 m in 20 m ( the distance between the two pegs ) . this is the amount and direction of collimation error .

(d) without moving the instument from C the line of sight must be adjusted down until it is horizontal . to do this  one must compute the reading (a4) on staff A horizontal sight from C , distance 30 m away , would give . 

by simple proporatin , as the error in 20 m is 0.500 , the error in 30 m = ( 0,500 ×30)/20 = 0.750 m. 

therefore the required reading at a4 is 3.500 - 0.750 m .

(e) (i) using the tilting screw ' tilt the telescope until it reads 2.750 m on the staff (ii) this movement will cause the tubular bubble to go off centre. Re - centre it by means of its adjusting screws will permit the raising or lowering of one end of the bubble .

the whole operation may be repeated if thought necessary.

the above process has been dealt with in great detail , as collimation error is one of the main sources of error in the leveling process.

the diagrams and much of the above detail can be dispensed with if the following is noted ;

(1) (H false - H true  ) = the amount of collimation error .

(2) if H false > H true then the line of sight is inclined up and vice versa .